This invention relates to an apparatus in the form of a test strip, which is useful for determining the presence of one or more components in a fluid sample. The test strip can be used by itself or in conjunction with an associated holding assembly.
The chemical analysis of fluids, including body fluids such as blood, serum, urine and the like; water; fluid food stuffs; etc. is often desirable and frequently necessary. Safety concerns, medical diagnosis, forensics, drug testing and other fields rely on determinations, either qualitative or quantitative, of the components of fluids. These determinations, or assays, must be rapid, reliable and accurate.
A major field of chemical analysis is devoted to "dry chemistry" determination of components and liquid samples. This refers to an apparatus that is dry to the touch. Generally, these apparatus take the form of monolayer and multi-layer test strips and other analytical test elements. These analytical test elements have excellent storage and handling properties, are convenient to use and provide accurate and reliable results.
The determination of a component, for example an analyte, in a fluid sample typically involves reacting the analyte with a binder. The binder undergoes some type of change following the reaction, which leads to a detectable signal. While the change may be caused directly by a reaction with the analyte, the change can also, and usually does, result from some property produced by the interaction between the analyte and the product of the reaction, where the interaction does not exist in unreacted components.
Many types of assays have been developed that utilize the above principles. One important type is the immunoenzymometric assay. This test involves the binding of an analyte of interest with a reaction or binding partner, where the binding partner carries a label. The binding partner is contained in a test strip or other apparatus so that it is non-reactive unless and until its partner analyte contacts the test strip. When this happens, the analyte and labelled binding partner bind to each other, forming a complex. This is accomplished by reacting the label carried by the binding partner with another substance, to form a detectable signal. When the label is an enzyme, as it frequently is, the substance is a substrate for the enzyme. The substrate for the enzyme either forms a visible color or changes color. Measuring the change or amount of color provides a measure of the produced complex, and hence of the analyte.
A problem with the above system is that one must have a sufficient amount of label binding partner to bind essentially all of the analyte in the sample. However, the amount of analyte is generally unknown. Thus, it is necessary to provide excess amounts of labelling binding partner, some of which will not react with the analyte, but carries the label. It nonetheless forms a detectable signal. Thus, unless one separates reacted label carrier from the unreacted portion, no readily perceivable test result can be achieved.
An additional feature of the immunoenzymometric assay is that after the sample has been contacted by the labelled binding partner and some of the partner has been bound to form the complex, the mixture of the complex and unreacted labelled binding partner contacts a sample of solid phase bound analyte or an analyte analog. This analog binds to the uncomplexed labelled binding partner. One can then make a clean division of the labelled partner bound to the analyte and excess labelled binding partner bound to the solid phase. The addition of substrate to either of these gives a color, so a determination can be made.
The immunoenzymometric assay is not the only type of assay used for an analytical system. For example, a competitive assay uses a sample of labelled analyte corresponding to the analyte to be determined, rather than labelled binding partner. A solid phase bound reactant is the binding partner for the analyte and the labelled analyte. If any of the analyte is in the sample that is tested, competition for the binding sites ensues. One then measures the amount of label either in the solid phase or the liquid phase, in the same manner described above for the immunoenzymometric assays, to determine the analyte.
In a displacement assay, a labelled analyte is already bound to a solid phase. When the sample contacts a test strip containing the bound labelled analyte, some of the labelled analyte will be displaced by the binding between the sample analyte and the solid phase bound binding partner.
A sandwich assay is another analytical system for carrying out a chemical analysis. Sandwich assays encompass a broad range of assay types; however, most refer to a formation of a complex between the analyte to be determined, a labelled epitopically active first antibody (Ab.sub.1 .cndot., where .cndot. indicates that the antibody is a labelled antibody), and a nonlabelled second antibody (Ab.sub.2), such as a whole monoclonal second antibody (mAb) or fragment thereof. The sandwich, which forms when the analyte contacts a test strip containing the diffusible second antibody Ab.sub.2 and first antibody Ab.sub.1 .cndot. is Ab.sub.2 --An--Ab.sub.1 .cndot. The sandwich then contacts a solid phase containing another antibody, which binds to the second antibody, but not to the first antibody. The result of this is separation of the complexed first antibody from the uncomplexed first antibody, which permits a determination in the same manner as discussed above.